(1) Field of the Invention
The present invention relates to a sealed battery and a manufacturing method for the same, and in particular to technology for properly welding an electrode terminal to a current collector in the manufacturing process.
(2) Description of the Related Art
In recent years, mobile electronic devices, such as mobile phones and personal digital assistants (PDAs), have been disseminated rapidly.
In these electronic devices, sealed batteries, such as nickel-metal hydride (Ni-MH) batteries and lithium-ion batteries, are frequently used as high-energy density power sources.
Generally, a sealed battery is formed by storing an electrode assembly and electrolyte into a rectangular external casing with a closed bottom, then sealing an opening in the external casing with a sealing plate (note, the shape of the external casing is not limited to a rectangular shape). The electrode assembly is formed, for example, by rolling a negative electrode plate and a positive electrode plate that have been layered with a belt-like separator sandwiched therebetween. The negative electrode plate is connected via a lead tab to a current collector. The current collector is connected to an electrode terminal (negative electrode terminal) provided in a sealing assembly. The positive electrode plate is connected via another lead tab to the external casing that also functions as a positive electrode terminal.
A manufacturing process for a sealed battery is described in, for example, Japanese Laid-Open Patent Applications No. 2003-272604 and No. 2001-291506. The following is one example of methods described in these documents. To begin with, the electrode assembly is stored into a rectangular external casing that is made of nickel-plated steel. As shown in cross-sectional views of FIGS. 5A-5D, a sealing plate 160, spacer 20, current collector 22, etc. are overlayed in listed order, then the layer is penetrated by a shaft portion 181 of an electrode terminal element 18x via a gasket 17 (FIGS. 5A-5D). Here, the shaft portion 181 penetrates through the current collector 22 by passing through a pass-through 220 provided in the current collector 22, in such a manner that an apical portion 187 projects out from the current collector, the apical portion 187 being provided in a first end of the shaft portion 181 in the penetration direction. Once the shaft portion 181 penetrates through the current collector 22, the apical portion 187 of the electrode terminal element 18x, which has an opening 185, is expanded diametrically to form a plate-like portion 182x by flaring the opening 185 with a jig inserted thereinto.
Next, the plate-like portion 182x is pressed against the current collector 22 in a plurality of steps (here, in two steps), such that a main surface C′ of the plate-like portion 182x comes into contact with a main surface B of the current collector 22 (FIGS. 5C-5D). In a case where the plate-like portion 182x has come into contact with the current collector 22 only by getting pressed thereagainst, the electrical resistance of the battery becomes unstable from the infiltration of electrolyte or the like into between the plate-like portion 182x and the current collector 22. For this reason, a laser beam is applied to a part of the pressed area to weld the plate-like portion 182x to the current collector 22, the laser beam being emitted from the proximity of the plate-like portion 182x and perpendicular to its another surface E which is exposed to an external space and is farther away from the current collector 22 (FIG. 6A).
Then, the lead tab extending from the electrode assembly is connected to the current collector 22. The sealing plate is fit into and welded to the opening in the external casing. Electrolyte is inserted inside the battery through a filler hole provided in the sealing plate. Finally, the battery is sealed by plugging the filler hole with a sealing plug.
However, there is a problem in the conventional manufacturing process for a sealed battery: deficiencies in battery performance may arise due to insufficient welding between an electrode terminal and a current collector, as will be described below.
As shown in an area S1 of FIG. 6A, the plate-like portion 182x has a slanted side 184x. The slanted side 184 makes an acute angle θ1 with the main surface B of the current collector 22. Here, in order to properly perform laser-welding, it is necessary to partially melt the plate-like portion 182x and the current collector 22 by applying a laser beam to the proximity of a perimeter of an interface between the plate-like portion 182x and the current collector 22.
Assume a case where a laser beam is applied in the area S1 so as to weld the plate-like portion 182x to the current collector 22, the laser beam being emitted from the proximity of the plate-like portion 182x and perpendicular to its surface E. The problem here is that the slanted side 184, as well as a part of the main surface B that is surrounded by the slanted side 184, is not exposed to the laser beam because a perimeter of the surface E blocks the laser beam.
This may give rise to the following problems: (i) only the proximity of the slanted side 184x melts from receiving thermal energy provided by the laser beam, leaving the current collector 22 and the plate-like portion 182x unmerged; and (ii) due to the interspace between the slanted side 184x and the part of the main surface B that is surrounded by the slanted side 184x, the perimeter of the surface E intensively receives the thermal energy from the laser beam and eventually gets sputtered and comes off from overheat, as shown in FIG. 6B.
Such problems caused by poor welding could not only damage the stability of electrical conductivity between the electrode terminal and the current collector, but also cause the sputtered, fallen portion to enter inside the electrode assembly and trigger a short circuit. These problems should thus be solved immediately so as to achieve a proper battery performance.